The present invention relates generally to titanium dioxide pigments and, more particularly, to a titanium dioxide pigment containing hydrous silica and alumina and which can be used for opacifying latex paints and other organic coatings. More particularly, it relates to that class of such pigments sufficiently weather-resistant for use outdoors. Still more particularly, it relates to a TiO.sub.2 pigment that possesses opacifying and weathering properties which allow it to be used economically in both interior and exterior paints.
Techniques for treating particulate rutile or anatase titanium dioxide with hydrous oxides such as those of silicon and aluminum to achieve either high opacifying power or outstanding weather-ability in coatings are well known. The application of hydrous silica to improve opacity is taught, for example, in U.S. Pat. Nos. 3,591,398 which issued to A. H. Angerman on July 6, 1971, and 3,410,708 which issued to W. J. McGinnis on Nov. 12, 1968. Typically, hydrous silica is applied in voluminous form by the addition of acid to an aqueous slurry of titanium dioxide containing sodium or potassium silicate. The application of hydrous silica so as to impart a marked improvement in weather-resistance is taught, for example, in U.S. Pat. Nos. 3,437,502 which issued to Alfred J. Werner on Apr. 8, 1969, and 2,885,366 which issued to R. K. Iler on May 5, 1959, and which describe the precipitation of a dense, amorphous silica coating by the slow addition of acid to a titanium dioxide slurry containing a soluble alkali-metal silicate.
The hiding or opacification exhibited by a titanium dioxide pigment in a paint film results from the scattering of light by the anatase or rutile particles. Particles which are crowded together in the film tend to scatter less efficiently, producing less opacity per unit of pigment than would be the case if the particles were so spaced apart that they could act in a more independent fashion. A voluminous silica treatment acts as a spacer to hold the pigment particles in such a favorable position, thereby enhancing their hiding efficiency.
The chalking of paint films during weathering is believed to be caused by the interaction of the organic binder or paint vehicle with the titanium dioxide pigment under the influence of light, particularly light in the ultraviolet region of the spectrum. This interaction causes decomposition of the paint film, thereby releasing the pigment particles which then appear as a white "chalk" upon the painted surface. The application of a dense amorphous silica substantially encapsulating each pigment particle creates an obstruction or barrier that isolates the titanium dioxide from physical contact with the organic binding medium, thereby diminishing the rate of photochemical degradation of the paint film.
Unfortunately, the voluminous silica coating which promotes better spatial relationships among the titanium dioxide particles and improves their hiding efficiency does not diminish contacts between the titanium dioxide and binder to a sufficient degree to impart acceptable resistance to the effects of weather. Further, the dense silica coating which imparts weather-resistance does not provide sufficient inter-particle spacing to achieve a high level of hiding efficiency.
Since it is desirable to maximize both the hiding efficiency and the weather-resistance of TiO.sub.2 pigments used in paint films, a number of attempts have been made to achieve that end. One approach relies on the application of a heavy coating or coatings of dense silica followed by a treatment with hydrous alumina to improve the wettability and dispersibility of the pigment particles. Heavy coatings of dense silica, however, are not as beneficial since, when applied to titanium dioxide slurried at practical concentration, they tend to cement the particles into aggregated structures which resist disintegration during the final milling of the pigment and its subsequent dispersion into paint. Pigments thus treated tend to exhibit a high oil absorption and a browner than optimal tint tone when evaluated by procedures such as those described in U.S. Pat. No. 3,342,424 which issued to W. R. Whately et al. on Sept. 19, 1967.
Paints designed for exterior use are often formulated to erode or chalk at a pre-determined rate so that a cleansing action occurs as the binder disappears. Aggregates released more slowly from the weathered surface tend to collect and hold dirt thus rendering the surface unsightly.
A second approach to TiO.sub.2 pigments exhibiting both excellent hiding efficiency and good weather-resistance has been the application of a lesser coating of dense silica followed by a voluminous treatment of hydrous silica-alumina mixtures. This approach provides for the desired weatherability at suitable levels of oil absorption and tint tone, however it does not insure the requisite hiding efficiency.
Still another approach is described in U.S. Pat. No. 3,649,322 which issued to W. Foss on Mar. 14, 1972 which teaches a titanium dioxide pigment encapsulated with a dense coating of aluminum silicate and hydrous alumina. Although this treatment affords good weatherability, such pigments tend to exhibit a high oil absorption and a browner than optimal tint tone.
Against the foregoing background, it is a primary objective of the present invention to provide a titanium dioxide pigment which possesses a high hiding efficiency and a weathering behavior that provides for a balance of durability with cleansing of the film.
It is another objective of the present invention to provide such a titanium dioxide pigment which may be used in both interior and exterior paints.
It is a further objective of the present invention to provide a titanium dioxide pigment treated with a two stage silica and an alumina coating.
It is yet still another objective of the present invention to provide a process for the surface treatment of titanium dioxide pigments with a two stage silica and an hydrous alumina surface treatment.
It is further still another objective of the present invention to provide a titanium dioxide pigment treated with two stage silica and two stage alumina coatings.